A thermal reactor typically comprises a reaction chamber which is sealed from the chamber's ambient environment. The reaction chamber might comprise mechanical seals at some places, e.g., at the position of an openable door which allows feeding one or more substrates into the reaction chamber, or at a position where a gas feed tube or gas exhaust tube is connected to the chamber. In chambers in which substrates are rotated during processing, the feed-through of a rotation mechanism for rotating a substrate during processing might also comprise a mechanical seal.
While these mechanical seals are preferably gas-tight to minimize process degradation due to interactions with the ambient environment, some of these seals may not be hermetic due to the high temperatures that are locally present at and around the seals. Purging of such seals with an inert gas can be performed to prevent ambient gas from entering the reaction chamber or to prevent process gases from exiting the reaction chamber at an undesirable location. Irrespective of the other gases used in processing, nitrogen gas has typically been used as the purge gas for seals, for practical and economic reasons, e.g., nitrogen gas is inert, readily available and relatively inexpensive.
It has been found, however, that this purging of the seal can itself cause undesirable complications. For example, purging can result in dilution of process gases with the purge gas. Moreover, this dilution might not be uniform over the entire volume of the reaction chamber, resulting in non-uniformities in processing results which vary depending upon location within the reaction chamber.
For example, non-uniform thermal oxide films can arise when inert gas is used to purge the rotational feed-through of a boat rotation mechanism for a reaction chamber that is part of a vertical furnace. These vertical furnaces typically comprise a door at the lower end, and the feed-through for boat rotation is typically provided in the door. An exhaust for process gas is typically also provided at the lower end of the furnace. It has been observed that when the feed-through is purged with nitrogen during an oxidation process using oxygen, the oxide thickness and uniformity is substantially affected and can vary by location within the furnace, e.g., from wafer to wafer and from one point to another on a single wafer.
Accordingly, there is a need for methods of purging a seal of a thermal reactor that does not significantly detrimentally affect the process performed within the reactor.
Thus, in accordance with one aspect of the invention, a mechanical seal at a downstream end of a thermal reactor is purged with a purging gas, wherein the selection of the purging gas depends on the density of the process gas and the position of the seal. When the seal is positioned at a lower end of the reactor, a purging gas is selected that is heavier than the process gas used. When the seal is positioned at an upper end of the reactor, a purging gas is selected that is lighter than the process gas used. Preferably, a gas exhaust is located at about the same end of the reactor as the seal. By “heavier,” it is meant that the purge gas has a molecular weight that is greater than the molecular weight of the process gas. Conversely, by “lighter,” the reverse is meant; that is, the purge gas has a molecular weight that is smaller than the molecular weight of the process gas.
In accordance with another aspect of the invention, a method is provided for semiconductor processing. The method comprises providing a semiconductor reactor having a reaction chamber having a mechanical seal and a gas exhaust. The mechanical seal is formed by interfacing parts of the reactor. An overpressure of purge gas is generated around the seal in a volume outside the chamber. The overpressure is an overpressure relative to the gas pressure inside the reaction chamber. The purge gas is chosen such that the purge gas remains, under the force of gravity or buoyancy, at a location proximate the mechanical seal if the purge gas enters the reaction chamber.
In accordance with yet another aspect of the invention, a vertical semiconductor reactor is provided. The reactor comprises a plurality of walls delimiting a reaction chamber. An end wall is proximate an end of the reaction chamber and comprises a mechanical seal. A surface is proximate the seal and at least partially delimits a volume surrounding the seal on a side of the end wall outside the reaction chamber. The reactor further comprises a purge gas inlet opening to the volume and in gas communication with the seal. The purge gas inlet is configured to discharge a purge gas into the volume and is connected to a purge gas source of purge gas having a purge gas molecular weight. The reactor also comprises process gas inlet for discharging process gas into the reaction chamber. The process gas inlet is connected to a process gas source of process gas having a process gas molecular weight. The process gas and purge gas are selected such that the purge gas is biased by gravity and buoyancy to the end of chamber proximate the seal.